Cores or paper tubes are produced generally by helically winding paper tape in layers around a mandrel in the form of a round rod to form a tubular body and continuously delivering the tubular body from the forward end of the mandrel. Two-pulley and three-pulley devices shown in FIGS. 12 and 13, respectively, are conventionally used for winding paper tape around the mandrel.
The tape winding device 2 shown in FIG. 12 has two rotatable pulleys 13, 13 arranged on opposite sides of a mandrel 12. A belt 29 obliquely wound on the mandrel 12 by one turn is endlessly reeved around the two pulleys 13, 13.
The tape winding three-pulley device 2 shown in FIG. 13 comprises one main rotatable pulley 13 disposed on one side of a mandrel 12, and first and second two auxiliary rotatable pulleys 13a, 13b arranged on the other side of the mandrel. A first belt 29 is endlessly reeved around the upper portion of the main pulley 13 and the first auxiliary pulley 13a, and a second belt 29a is endlessly wound around the lower portion of the main pulley 13 and the second auxiliary pulley 13b. The first and second belts 29, 29a are wound on the mandrel 12 by one turn symmetrically.
With these two-pulley and three-pulley devices, paper tapes 91a, 91b paid off from two tape feeders 90, 90, respectively, are engaged between the mandrel 12 and the belt 29 or belts 29, 29a driven by the pulleys and helically wound on the mandrel 12 into a paper tube 92 while being slipped on the mandrel by the friction between the tape and the belt. Thus, the belt or belts function to wind the paper tapes around the mandrel and also to forward the paper tube axially thereof.
However, when the belt is tensioned as wound around a mandrel of small diameter as stated above and is driven in this state at a high speed for producing a paper tube of small diameter, the mandrel of small diameter warps in an undulating fashion, rendering the paper tube no longer smoothly rotatable and movable axially thereof and making it impossible to obtain a paper tube of commercial value. With the conventional paper tape winding device, therefore, there arises a need to lower the tension on the belt and to drive the belt at a reduced speed. Consequently, when producing a paper tube, for example, having a small inside diameter of 6 mm and a wall thickness of 2 mm, the device is as low as about 2 m/min in production speed and is very low in production efficiency.
Further regardless of whether paper tubes have a small diameter, medium wall thickness or large wall thickness, those for a particular use are required of very high pressure resistance. To impart improved pressure resistance to the paper tube, the paper tape must be subjected to great back tension and tightly wound on the mandrel. Nevertheless, if the belt wound on the mandrel by one turn is driven to wind the paper tape around the mandrel and also to forward the resulting paper tube as in the conventional practice, slippage is likely to occur between the paper tube and the tape, whereas increased back tension on the paper tape permits slippage of the belt. Since the torque of the pulley is transmitted to the tape by virtue of the friction between the pulley and the belt and also the friction between the belt and the tape, the power transmission efficiency is low. Consequently, even if an increased torque is given to the pulley, difficulty is encountered in winding the highly tensioned tape around the mandrel helically continuously. Thus, it is difficult to produce paper tubes having the desired pressure resistance.
Further when the belt is driven as wound on the mandrel obliquely, the belt acts to advance axially of the mandrel at the portion thereof around the mandrel, while the tension on the belt retracts the belt axially of the mandrel. Since this motion is repeated, the belt is always reciprocatingly moved over a small distance axially of the mandrel. Because the paper tape is wound on the mandrel by the belt reciprocatingly moving in this fashion, a clearance is liable to occur between the adjacent two portions of the tape wound on the mandrel. Such a clearance not only impairs the strength of the paper tube but also varies the speed of axial transport of the tube.
Before the paper tube is made into a finished product, the tube is not infrequently subjected to aftertreatments such as coating of the tube surface with resin and grinding of the resulting surface. However, the conventional machine for producing paper tubes is low in its force to deliver the paper tube axially thereof, so that such aftertreatments offer increased resistance to the axial movement of the tube to result in variations in the speed of axial movement of the tube and make the tube production speed unstable, if the treatments are conducted in sequence by devices connected to the production line downstream from the machine. Consequently, the aftertreatment devices are not in smooth operative relation with the machine, failing to produce products of good quality. With the conventional device, therefore, the paper tube is cut to a specified length, and the cut tube is then treated as required by another processing line, hence a poor production efficiency.